A procedure of solid-liquid separation (clarification process) to remove suspended substances is indispensable for clear water treatment to obtain drinking water or to obtain industrial water from natural water sources such as river water, lake and marsh water and underground water, and for sewage water treatment to obtain regenerated water for miscellaneous uses or to obtain dischargeable clean water by treating domestic wastewater such as sewage water. There is a major need for clarification procedure to remove suspended substances (such as clay, colloid and bacteria) derived from water from natural water sources in the case of a clear water treatment, and to remove suspended substances in sewage water or suspended substances (such as sludge) in biologically treated (secondary treated) water with activated sludge and the like in the case of a sewage water treatment. Conventionally, sedimentation method, sand filtration method, and coagulation sedimentation plus sand filtration method have been predominantly used for these clarification processes, but recently a membrane filtration method has become most common. Merits of the membrane filtration method include: (1) clarification level in resultant water quality is higher and stable (enhanced safety of resultant water), (2) installation space for filter unit is small, and (3) an automated operation is easier, etc. For example, in a clear water treatment, a membrane filtration method is employed as a substitute for coagulation sedimentation plus sand filtration or as a means to further improve the water quality of treated water which has undergone coagulation sedimentation plus sand filtration by arranging it after the coagulation sedimentation plus sand filtration. In the case of a sewage water treatment, studies regarding the use of the membrane filtration method for, for example, sludge separation from secondary treated water of sewage water etc., are also in progress.
In these clarifying procedures by membrane filtration, a hollow fiber-like ultrafiltration membrane or microfiltration membrane (with pore diameters in the range from several nm to several hundred nm) is mainly used. Filtration systems using a hollow fiber-like filtration membrane include two types, an internal pressure filtration where water is filtered from inner surface side toward outer surface side of the membrane and an external pressure filtration where water is filtered from outer surface side toward inner surface side of the membrane. The external pressure filtration is, however, advantageous because it enables membrane surface area on the side in contact with raw suspended water to be larger, so that load of suspended substances per unit area of membrane surface can be less.
Clarification by a membrane filtration method is prevailing in the clear water treatment and the sewage water treatment as an alternative or a complementary technique for conventional ones due to the above described many advantages which a conventional sedimentation or sand filtration method does not have. However, wider spread of a membrane filtration method is hindered because a technology enabling a long-term stable operation of membrane filtration has not been established (see, Y. Watanabe, R. Bian, Membrane, 24(6), 310-318 (1999)). The most common hinderance to a stable operation of membrane filtration is the deterioration of the permeability of a membrane. The first cause of the deterioration of permeability is clogging of a membrane by suspended substances and the like (fouling) (see, Y. Watanabe, R. Bian, Membrane, 24(6), 310-318 (1999)). In addition, a membrane surface may be abraded by suspended substances to cause the deterioration of permeability.